1. Field of the Invention
The present invention relates to a ring-mounted microwave device with two bridge-mounted diodes of the switch or amplitude modulator type with a wide pass band, the performance of which is made independent of the frequency. This aim is achieved by neutralizing the reactive currents of the diodes, due to the capacitances of the diodes when they are off. The reactive currents are neutralized by two capacitors mounted diagonally with respect to the bridge, each of them being connected between the anode of a diode and the cathode of the other diode, the set thus constituting a balanced ring.
2. Description of the Prior Art
Devices such as a diode operated modulator or switch, used in the microwave range from 0.5 to 2 GHz, for example, are well known. An example of a modulator is given in FIG. 1. This modulator comprises, between an input 1 of the circuit and an output 10, a first powering stage, symmetrical with the diodes, which may consist of a micro-coiled transformer consisting of a primary winding 2 and two secondary windings 3 and 4. These two secondary windings supply two diodes 5 and 6, mounted in the same direction, which are themselves connected to two primary windings 7 and 8 of an output transformer, the secondary winding 9 of which powers the output 10. The diodes 5 and 6 are polarized from two midpoints 11 and 12 taken between the windings 3 and 4, on the one hand, and 7 and 8, on the other hand.
Other modulators are known. These known modulators use either a single diode instead of two diodes, or they are made with a three-plate or microstrip technology; This fact in no way alters the problem to which the invention provides a solution.
The problem that arises with known modulators is that, when the diodes are off, they are equivalent to a capacitor which has been represented by a capacitor 13, for the diode 5 and a capacitor 14, for the diode 6, shown in dashes. However, it is noted that the isolation obtained, for a switch, or the attenuation obtained, for a modulator, diminishes when the frequency increases. This phenomenon is shown in FIG. 2 where it is seen that the isolation, expressed in negative decibels, diminishes when the frequency increases, in the GHz range. This phenomenon is obviously due to the capacitance of the diodes in the off state.
The isolation or attenuation obtained with zero current is optimum if the output current of the device is zero, i.e. if the current given out by each diode is zero. Now, the off state of each diode displays a certain capacitance which is subjected to a high frequency electrical voltage: the current of each diode is not zero.
According to the invention, a current in phase opposition is set against the current of the first diode. This current in phase opposition is picked up before the second diode by means of a capacitance of substantially the same value as the capacitance displayed by a diode which is off: the two currents cancel each other out. Since this assembly is symmetrical, the resulting current is also zero.